The Trace Metal Biogeochemistry Lab at USC
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Specific Projects
Iron, copper and denitrification
I work with Bess Ward, a microbiologist at Princeton University, to study iron/copper interactions with denitrifying bacteria in oxygen minimum zones (OMZs). We want to determine how, and the extent to which these metals influence the relative importance of anammox versus denitrification, and the accumulation of nitrous oxide in the ocean. I discovered that the secondary nitrite maximum in OMZs is associated with an intense band of Fe(II), a unexpected finding at odds with current thinking about the energetics of microbial processes. I hypothesize that denitrifiers or anammox, each with high Fe requirements, reduce Fe(III) to Fe(II) as part of an Fe-acquisition strategy allowing these microbes to succeed in regions where the concentrations of bioavailable iron immediately above and below their ecological niche is exceedingly low. The chemistry of iron within this zone is completely different than outside it, and creates an ideal, yet thermodynamically unstable, regime for these microbes. A major premise I am exploring in this research is that microbes contribute to the complexity of seawater chemistry, possibly through a quorum sensing mechanism, to fabricate a chemical regime that suits their physiological needs. This work has generated controversy, since it goes counter to prevailing thought about microbial thermodynamics and Fe acquisition, but I have documented it extensively in all three of the world’s OMZs, including a zonal section across the Arabian Sea.
Ward and I have discovered that while denitrifiers have an extraordinary ability to acquire copper from strong complexes in seawater (needed for some forms of nitrite reductase and all forms of nitrous oxide reductase), these copper enzymes are highly vulnerable to various forms of reduced sulfur at realistic seawater concentrations. As a result, microbes occupying chemical niches across redox gradients are sensitive to chemical interferences from adjacent niches that disrupt their function, with significant biogeochemical consequences, for example nitrous oxide accumulation.
Copper Limitation of Nitrification
I am also actively investigating the relationship between copper geochemistry and nitrification. Copper is both required for ammonium monooxygenase, and it is associated with approximately 30% of the genome of ammonium oxidizing archaea (AOA), the ocean’s most important nitrifiers. Current results from my Dimensions of Biodiversity Project with Ginger Armbrust, Dave Stahl and others suggests that Cu-limitation may be an important factor in environments as diverse as the primary nitrite maximum in the eastern tropical South Pacific and the euphotic zone in Puget Sound.
Arabian Sea
Perhaps my favorite place to study marine geochemistry is the Arabian Sea. This is a highly biologically active region that plays a key role in the global nitrogen cycle. Moreover, its dynamics are thought to be highly vulnerable to climate change as it is surrounded on three sides by warming landmasses. My close colleague Wajih Naqvi, director of India’s National Institute of Oceanography (NIO) in Goa has worked with me on several cruises to the Arabian Sea. My group also participated on the Japanese GEOTRACES cruise though the Arabian Sea in 2009. Future cruises are planned in 2015 to establish a time series station in the Northeastern Arabian Sea using an Indian research vessel. Important findings include demonstrating for the first time the importance of Fe limitation in controlling primary production in the Arabian Sea and the identification of Fe enrichment and Cu depletion in the region’s oxygen minimum zone. Recently an Indian student, Jagruti Vedamati, completed her Ph.D. in my group. Her work included a major survey of various metals in the Arabian Sea. I am deeply committed to stronger ties with researchers at NIO and elsewhere in the Indian subcontinent.
GEOTRACES
U.S. GEOTRACES contributes to the mission of the GEOTRACES program, an international study of the marine biogeochemical cycles of trace elements and their isotopes. The mission, derived from the SCIENCE PLAN, is “to identify process and quantify fluxes that control the distributions of the key trace elements and isotopes in the ocean, and to establish the sensitivity of these distributions to changing environmental conditions”.
I have been involved with this program since its inception, including the intercalibration cruises in 2008 and 2009, and the two US GEOTRACES sections: the North Atlantic Zonal Transect (NAZT) in 2010 and 2011 and the Eastern Pacific Zonal Transect (EPZT) which I led in 2013.
In the NAZT, my student Jeremy Jacquot surveyed copper and copper speciation across the Atlantic from surface to seafloor. This resulted in a paper that was an important part of his Ph.D dissertation.
The abstract of his paper is pasted below, along with detailed sections of copper and copper speciation.
Copper (Cu) distribution and speciation were characterized along a meridional section in the North Atlantic Ocean from Lisbon, Portugal, to Woods Hole, Massachusetts as part of the U.S. GEOTRACES program. Dissolved Cu profiles displayed many of the same features identified by other researchers, including sub-surface scavenging and a linear increase with depth, but many also exhibited unique properties and geographic trends. Concentrations ranged from 0.43 nM at the surface to 3.07 nM near the seafloor. The highest concentrations were measured in deep waters to the west of Cape Verde and northwest of the Canary Islands while the lowest concentrations were measured in upper waters, mostly between Mauritania and Cape Verde. The westernmost sampling sites overlying or adjacent to the U.S. east coast continental shelf featured surface maxima that decreased in magnitude moving east toward Bermuda, reflecting declining inputs from Cu-enriched coastal waters and North American aerosols. Free Cu (Cu2+) concentrations were tightly controlled by organic complexation and scavenging across the section with values varying between 1.54 fM and 1.07 pM. These results provide the first evidence that Cu2+ concentrations are strongly complexed throughout the water column, even in boundary zones where dissolved Cu concentrations are elevated because of local sources. Strong organic ligands (L) acted as a buffer for Cu2+, restricting concentrations to a narrow range (10 – 100 fM) throughout most of the water column. Cu2+ and dissolved Cu were strongly scavenged by suspended particulate matter within several benthic nepheloid layers and a hydrothermal plume above the Trans-Atlantic Geotraverse (TAG) vent field, Mid-Atlantic Ridge (MAR).
